Psa 137:9 Happy shall he be, that taketh and dasheth thy little ones against the stones.

This is not a matter of revenge, but wanting justice for what the Babylonians had done to them. This was what conquerors did back then.

I do think it is kind of funny though that many use this scripture to vilify God--when these are not God's wishes, He is not the one who will be "happy". I think that when we have a mind bent against someone all we can see is evil in everything that is said--like in the case of Job's three supposed comforters. No matter what he said, they twisted into something evil on his part.

These words are the words and thoughts of the those taken into captivity by Babylon--they want justice for what was done to them and their children. They also know the fall of Babylon was also predicted and that is what they are looking forward to.

Are these sentiments any different than the parents of a child who was brutalized or killed--that they want the ones who did it to suffer the same?

The "happy" ones are the Medes and the Persians--they were just like the Babylonians and did the same thing when they conquered them. They were happy to do it.